1. Field of the Invention
The present invention relates to a light-emitting element drive circuit.
2. Related Background Art
An optical pickup is a device that is capable of reading information stored on the recording medium by monitoring the reflected light therefrom. In some cases, a laser diode is also employed for writing information on to a recording medium. Drive circuits for such laser diodes have previously been developed.
In an optical pickup, undesirable light reflected within the device enters the photodetector and becomes noise contained in the necessary information. Noise is also caused by instability of the light emission of the light-emitting element when light reflected by the storage medium returns to and enters the light-emitting element.
Techniques for driving the light-emitting element at high-frequency have therefore been considered in order to suppress the noise (mode-hop noise). High-frequency current is superimposed on the DC current component which is supplied to the light-emitting diode as the drive current. A suitable frequency for the high-frequency capable of being employed in current CD players is 300 to 500 MHz.
In order to lower power consumption, the possibility of not supplying such high-frequency current to the light-emitting element has also been considered. The average value of the high-frequency current itself also varies with temperature and deterioration over a period of years. When the average value of the high-frequency current varies in this way, the level of the drive current varies. When high-speed reading is performed, the decline of the light intensity increases the likelihood of occurrence of spurious detection being produced by small level variations. It is therefore preferable that level variation should be small.
In view of the above, an object of the present invention is to provide a light-emitting element drive circuit in which drive current is produced by superimposing a high-frequency current component on a DC current component and supplying this to the light-emitting element but wherein level variation of this drive current can be suppressed.
In order to solve the problem described above, in a light-emitting element drive circuit according to the present invention, in which, in first and second current mirror circuits having respective parallel lines, the respective one side of the lines is connected to the light-emitting element and respectively DC current and high-frequency current are passed to the respective other side of the lines in the first and second current mirror circuits, drive current resulting from the superposition of the high-frequency current on the DC current being supplied to the light-emitting element through the node of the connection, wherein the DC current component of the high-frequency current flowing through the second current mirror circuit is detected and the detected DC current component is arranged to be subtracted from the DC current flowing through the respective other side of the lines of the first current mirror circuit.
That is, when the DC current component of the high-frequency current in the second current mirror circuit increases, the average value of the drive current applied as the sum of the currents flowing through both of the one-side lines of the first and second current mirror circuits is increased.
When the DC current component of the high-frequency current flowing through the second current mirror circuit is increased, the amount of this increase is subtracted from the DC current flowing through the other-side line of the first current mirror circuit. Since the drive current results from the superposition, on the high-frequency current, of a DC current equal or proportional to the DC current flowing through the other side of the first current mirror circuit, variation of the average value of the drive current can be suppressed by subtraction of the amount of the aforesaid increase from the drive current.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.